The present invention relates generally to improving back-off efficiency and peak output power level for class-E outphasing power amplifiers (PAs).
Modern wireless communication systems require advanced modulation techniques that result in non-constant envelope modulation and very high peak-to-average power ratio (PAPR) in the modulated RF signal. For such signals, linear power amplifiers quickly become less power efficient as the amount of load current demanded by a load decreases. The term “power back-off” is used to indicate the situation or condition when the instantaneous load power is low relative to the peak power that can be supplied to the load by the PA (power amplifier). Many techniques have been proposed in the past to improve the power efficiency of the power amplifiers as the load current decreases. The architecture described in the article by N. Wongkomet, L. Tee and P. R. Gray entitled “A +31.5 dBm CMOS RF Doherty Power Amplifier for Wireless Communications”, IEEE Journal of Solid-State Circuits, Vol. 41, No. 12, pp. 2852-2859, December 2006 combines two power amplifiers biased in different operating point “regions” to reduce the power efficiency degradation. “Envelope tracking” (ET) and “envelope elimination and restoration” (EER) are two other efficiency improvement techniques which require a power supply control scheme. See the article by M. Hassan, L. E. Larson, V. W. Leung, D. F. Kimball and P. M. Asbeck entitled “A Wideband CMOS/GaAs HBT Envelope Tracking Power Amplifier for 4G LTE Mobile Terminal Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 60, No. 5, pp. 1321-1330, May 2012.
Outphasing, or linear amplification using nonlinear components (LINC), produces amplitude modulation of RF (radio frequency) output signals by combining the output of two power amplifiers that are driven by constant envelope (i.e., constant amplitude) phase modulated signals. Outphasing power amplifier configurations also have been reported using switching power amplifiers such as class D and class-E amplifiers. See the article by Frederick Raab entitled “Efficiency of Outphasing RF Power-Amplifier Systems”, IEEE Transactions on Communications, Vol. 33, No. 10, pp. 1094-1099, October 1985 and the article by T. Hung, D. K. Choi, L. E. Larson, P. M. Asbeck entitled “CMOS Outphasing Class-D Amplifier With Chireix Combiner”, IEEE Microwave and Wireless Components Letters, Vol. 17, No. 8, pp. 619-621, August 2007. The outphasing configuration of class-E power amplifiers (PAs) with an asymmetric transmission line combiner is shown in Prior Art FIG. 1A. See the article by R. Beltran, F. H. Raab, A. Velazquez, “HF outphasing transmitter using class-E power amplifiers,” IEEE MTT-S International Microwave Symposium, pp. 757-760, June 2009.
“Prior Art” FIG. 1A shows an outphasing transmitter which produces a variable amplitude output by varying the phases of the driving signals to its RF-power amplifiers. The phase modulation causes the instantaneous vector sum of outputs of the two PAs to follow a desired signal envelope amplitude. Outphasing is attractive because signal phase can easily be modulated over a wide bandwidth, and constant envelope signals on individual paths allow the use of switch mode power amplifiers which have higher efficiency than linear power amplifiers. In a microwave implementation, power combiners based upon transmission lines are often used. The outphasing transmitter, also known as a “linear amplification using non-linear components” (LINC) transmitter, was originally developed to provide linear amplification with active devices that have poor linearity. Chireix added complementary shunt reactances at the inputs of the combiner to improve the efficiency at certain power back-off levels.
Prior Art FIG. 1A also shows a vector diagram to illustrate a conventional way of generating the phase modulated RF drive signals S1(t) and S2(t) that can be used in the subsequently described embodiments of the present invention. FIG. 1A generally indicates how the drive signals S1(t) and S2(t) are generated in a basic outphasing system. (The basic outphasing technique is described in the above mentioned article “Efficiency of Outphasing RF Power-Amplifier Systems” by F. Raab.) In FIG. 1A, drive signals S1(t) and S2(t) are generated in response to the incoming amplitude and phase modulated signal S(t) represented by the vector 17C. Drive signals S1(t) and S2(t) are represented by vectors 17A and 17B, respectively. Specifically, in the outphasing power amplifier the incoming signal with amplitude and phase modulationS(t)=a(t)ejϕ(t) is decomposed into two constant envelope phase modulated signals S1(t) and S2(t) on conductors 14A and 14B in FIG. 1A, respectively, asS1,2(t)=Aej[ϕ(t)±θ(t)]whereθ(t)=cos−1[a(t)/(2A)]and the constant amplitude A is defined as the maximum of a(t)/2, where ϕ(t) is the phase of the original amplitude and phase modulated signal S(t). The constant amplitude envelopes of signals S1(t) and S2(t) allow the use of switched-mode implementations of power amplifiers 3A and 3B, respectively.
Referring next to Prior Art FIG. 1B, basic class-E switching power amplifiers consist of a capacitor shunting the transistor, a series tuned load network, and a choke inductor as shown. The term “class E” refers to a tuned power amplifier composed of a single-pole switch and a load network. The switch consists of a transistor M that is driven ON and OFF at the carrier frequency of the signal to be amplified. In its most basic form, the load network consists of a resonant circuit in series with the load, and a capacitor which shunts the switch. The total shunt capacitance is due to the output capacitance that is inherent in switching transistor M plus any extra capacitance coupled in parallel to the switch. The drain voltage waveform on conductor 11 is then determined by the switch transistor M when it is ON, and by the transient response of the load network when switch transistor M is OFF. See the article Frederick Raab entitled “Idealized operation of the class E tuned power amplifier”, IEEE Transactions on Circuits and Systems, Vol. 24, No. 12, pp. 725-735, December 1977.
Outphasing power amplifier configurations have been reported using switching power amplifiers such as class-D and class-E switching power amplifiers. See the article by T. Hung, D. K. Choi, L. E. Larson, P. M. Asbeck entitled “CMOS Outphasing Class-D Amplifier With Chireix Combiner”, IEEE Microwave and Wireless Components Letters, Vol. 17, No. 8, pp. 619-621, August 2007 and the article the article R. Beltran, F. H. Raab, A. Velazquez, “HF outphasing transmitter using class-E power amplifiers,” IEEE MTT-S International Microwave Symposium, pp. 757-760, June 2009.
Commonly assigned published patent application Pub. No. 2013/0210376 entitled “LINC Transmitter with Improved Efficiency” by Hur et al., published Aug. 15, 2013, discloses a LINC transmitter including class-D power amplifiers with combiner circuitry having improved efficiency.
Conventional or “traditional” RF power amplifiers suffer from loss of power efficiency, i.e., power delivered by the amplifier to the load divided by total power consumed by the power amplifier, as the amount of instantaneous load power decreases. In handheld devices, low power efficiency of the RF power amplifiers causes shorter battery lifetimes, and in base station applications the low power efficiency of the RF power amplifiers may result in wasted power and high heat sinking costs.
A problem of the prior art is that the need for high data rates and efficient spectrum utilization in modern wireless communication systems results in high peak-power-to-average-power ratios of modulated signals therein. This requires associated RF power amplifiers to operate much of the time at greatly reduced output power levels, corresponding to high levels of “power back-off” operation.
Another problem of the prior art is achieving high output power levels from the power amplifiers without exceeding transistors' maximum voltage specifications in the integrated circuit.
Techniques for adding a third harmonic component and other harmonic components to the signal on the drain terminal of a switching transistor of a class-E power amplifier for the case of a non-outphasing “static power amplifier” is known. See the article by S. D. Kee, I. Aoki, A. Hajimiri, D. Rutledge entitled “The class-E/F family of ZVS switching amplifiers”, IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 6, pp. 1677-1690, June 2003. The prior techniques show addition of harmonic components to allow switching operations, as in class-E power amplifiers, with reduced peak drain voltage. Although the foregoing Kee et al. article discloses using additional third harmonic signal components to shift the drain voltage of a class-E power amplifier, it discloses use of this technique only in a single static (i.e., for a single value of phase angle θ) class-E power amplifier, by adding third harmonic component to shift the amplifier switching transistor's drain voltage.
Thus, there is an unmet need for a way of increasing the power efficiency of a class-E outphasing power amplifier.
There also is an unmet need for a way of increasing the power efficiency of a class-E outphasing power amplifier over a range of power back-off conditions.
There also is an unmet need for a way of increasing the peak output power of a class-E outphasing power amplifier by increasing the amplifier's power supply voltage without violating transistor reliability limits.
There also is an unmet need for a way of increasing the power efficiency over a range of power back-off conditions and the maximum peak output power of a class-E outphasing power amplifier by increasing the amplifier's power supply voltage without violating transistor reliability limits utilizing various alternative combiner circuit configurations.
There also is an unmet need for a way of providing simplified combiner circuitry for a class-E outphasing power amplifier.
There also is an unmet need for an improved way of providing a class-E outphasing power amplifier without use of a quarter-wavelength transmission line combiner.